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1 1 THE ST. MICHAEL SCHOOL THIRD FORM CHEMISTRY MANUAL 3 SYMBOLS AND FORMULAE, CHEMICAL BONDING AND CHEMICAL EQUATIONS COMPILED BY G.WALKER, L.WORRELL, T. HARDING REFERENCE BOOKS Anne Tindale Chemistry A Concise Revision Course For CXC A.M. Hughes Chemistry in Balance OBJECTIVES SYMBOLS AND FORMULAE 1. Write the correct symbols and valencies of the first twenty elements of the periodic table as well as the other common elements, barium, bromine, iodine and the transition metals, iron, copper, silver, zinc, nickel and lead. 2. Write correct symbols and valencies for the radicals, sulphate, carbonate, nitrate, hydrogen carbonate, hydrogen sulphate, nitrate, phosphate and ammonium. 3. Write the correct formula of an ionic substance based on the valencies of the ions involved. CHEMICAL BONDING 1. Explain the formation of ionic and covalent bonds. 2. Predict the likelihood of an atom forming an ionic or covalent bond based on its structure 3. Write formulae to represent ions and molecules. 4. Draw bonding diagrams (dot and cross or otherwise) to show the type of bonding in molecules and compounds. 5. State the properties of ionic and covalent compounds. CHEMICAL EQUATIONS 1. Write balanced equations to represent chemical reactions

2 2 SYMBOLS, VALENCIES AND WRITING CHEMICAL FORMULAE Symbol this is a shorthand way of describing atoms of different elements. Valency this is the combining number of an element or a group of elements. It is the number of electrons an atom has to lose, gain or share in order to achieve a stable electronic structure. The valency is used to work out the formula of a compound. The valency table below shows the symbols and valencies of the elements and radicals that you MUST YOU KNOW! VALENCY OF 1 VALENCY OF 2 VALENCY OF 3 VALENCY OF 4 Hydrogen, H Lithium, Li Sodium, Na Potassium, K Fluorine, F Chlorine, Cl Beryllium, Be Magnesium, Mg Calcium, Ca Oxygen, O Sulphur, S Boron, B Aluminium, Al Nitrogen, N Phosphorus, P Carbon, C Silicon, Si Bromine, Br Iodine, I Barium, Ba Silver, Ag Copper (I), Cu Copper(II), Cu Iron (II), Fe Lead (II), Pb Nickel, Ni Zinc, Zn Iron (III), Fe Lead (IV), Pb Ammonium, NH 4 + Nitrate, NO 3 Hydroxide, OH - Hydrogen carbonate, HCO 3 Hydrogen sulphate, HSO 4 - Sulphate, SO 4 2- Sulphite, SO 3 2- Carbonate, CO 3 2- Phosphate, PO 4 3- Points to note: In the first section of the valency table, we have the first twenty elements of the Periodic Table. Remember the valencies of Groups I, II and III elements (metals) are the same as the group that they are in. On the other hand, the valencies of the elements in Groups IV to 0 can be found by subtracting the group number from 8. These elements are non- metals. In the second section we have some elements that are outside of the short Periodic Table. In the third section we have the transition metals where most of them have a valency of 2. However, some transition metals can have more than one valency. When this is the case, the valency of the metal is indicated by bracketed capitalised roman numerals. For example, iron

3 3 can have a valency of 2 or 3, so it can be represented as Fe (II) or Fe (III), depending on the compound. In the fourth section we have the radicals (or compound ions). These ions are groups of nonmetals that act as a unit. Most of them are negatively charged and only one is positively charged. From the table on page 2, how many radicals must you know? A chemical formula represents the proportions, by mass, of the different elements in a compound. It is a description of a compound. A binary compound is composed of two different elements only. When a compound is made up of a metal and a non-metal, the metal always comes first, for example, sodium chloride. Also note that binary compounds containing: 1) oxygen are called oxides 2) sulphur are called sulphides 3) nitrogen are called nitrides 4) fluorine are called fluorides 5) chlorine are called chlorides 6) bromine are called bromides 7) iodine are called iodides 8) hydrogen are called hydrides WRITING CHEMICAL FORMULAE OF BINARY COMPOUNDS USING VALENCIES EXAMPLES STEPS EXAMPLE 1. Write the name of the compound Aluminium oxide 2. Write the symbols of the separate elements Al O 3. Write the valencies under the symbols Balance the valencies if necessary by finding the lowest common multiple of the formula. 2 x3 3x2 5. Write the formula. Al 2O 3 A simpler, less scientific method to work out the formulae of binary compounds is the swapping method where there is swapping of valencies. Place the valency of the first element after the symbol of the second, and place the valency of the second element after the symbol of the first. Aluminium Al oxide O 3 2 Formula: Al2O3

4 4 MORE EXAMPLES... Now copy carefully into the spaces below more examples as your teacher goes through them on the board. Write the chemical formulae for the following compounds. Sodium chloride Calcium oxide Aluminium nitride Zinc iodide Potassium sulphide Iron (III) oxide WRITING CHEMICAL FORMULAE USING RADICALS Please note: if a radical occurs MORE THAN ONCE in a formula, then it must be enclosed in brackets and the number written outside of the brackets at the bottom right. Also note that the charge on the radical is NOT included when writing compounds. Again, listen and copy carefully as your teacher goes through the following examples on the board. Sodium hydroxide Calcium hydroxide Ammonium phosphate Formula: Formula: Formula:

6 6 CHEMICAL BONDING The inert gas structure is a very stable structure. Group 0 elements all have full outer shells of electrons and hence are stable and unreactive. Noble gas configurations are He (2), Ne (2.8) and Ar (2.8.8). All the other atoms of elements in the periodic table want to imitate the noble gas nearest to it and hence achieve a stable configuration. They attempt to gain full outer shells and become stable by: losing electrons from their outer shell, or gaining electrons to their outer shell, or sharing electrons in their outer shell with other atoms. In doing this, atoms BOND with each other. There are two types of bonding that we will be concentrating on at this level. These two types of bonding are: Ionic or electrovalent bonding Covalent bonding Ionic or Electrovalent Bonding In ionic bonding a metal MUST be present in the formula. There is a TRANSFER of outermost electrons (valence electrons) from metal atom(s) to nonmetal atom(s) to form an ionic substance. Ions are formed in the process. The metal atoms formed cations and the non-metal atoms formed anions. Ions have full outer shells of electrons. The force of attraction between the oppositely charged ions is the electrovalent or ionic bond. DRAWING BONDING DIAGRAMS IN IONIC BONDING There are two methods of drawing bonding diagrams (1) dot and cross method and (2) electronic shell method. EXAMPLE #1 - SAMPLE QUESTION The atomic numbers of sodium and chlorine are 11 and 17 respectively. (a) Write the formula and give the name of the compound formed between sodium and chlorine. (b) Draw diagrams (dot and cross method and electronic shell method) to show the bonding between sodium and chlorine. (a) The electronic configuration of Na is since the atomic number is 11. The electronic configuration of chlorine is since the atomic number is 17. Sodium is in Group 1 since it has a one electron in its outer shell. It therefore has a valency of 1 since it is in Group 1. Chlorine is in Group 7 since it has 7 electrons in its outermost shell. It therefore has a valency of 1 (8-7=1). Elements Na Cl Valency 1 1

7 7 Therefore the formula is NaCl and the name of the compound is sodium chloride. NOTE: ALWAYS WORK OUT THE FORMULA FIRST BEFORE DRAWING BONDING DIAGRAMS! (b) Dot and Cross method (short method shows only the symbols and outermost electrons only) Electronic shell method (long method showing all the shells of the atoms) In the diagrams above, the sodium atom needed to lose 1 valence electron in order to become stable and the chlorine atom needed to gain 1 electron to its outer shell in order to obtain a full outer shell of electrons. The sodium atom then transfers its valence electron to the chlorine atom and as a result, both atoms become ions and hence stable. Since sodium ions and chloride ions are oppositely charged, they attract each other and form ionic bonds.

8 8 EXAMPLE # 2 The atomic numbers of magnesium and fluorine are 12 and 9 respectively. Draw bonding diagrams (both methods) to show the bonding between magnesium and fluorine. ALWAYS work out the formula FIRST! Magnesium has an electronic configuration of since its atomic number is 12. It is therefore in Group 2 and has a valency of 2. Fluorine has an electronic configuration of 2.7 since its atomic number is 9. It is therefore in Group 7 and has a valency of 1. Element Mg F Valency 2 1 Formula MgF 2 We therefore need one magnesium atom and 2 fluorine atoms for bonding to take place. Dot and Cross method EXERCISE: In your notebook show the electronic shell method for magnesium and fluorine.

9 9 PROPERTIES OF IONIC (ELECTROVALENT) COMPOUNDS (e.g. sodium chloride, NaCl) 1. They consist of ions. 2. They are usually crystalline solids. 3. They conduct electricity when melted (or molten) or when dissolved in water as the ions are free to move and carry the electricity. However, they DO NOT conduct electricity in the solid state as their ions are held together by strong bonds and are therefore not able to move. 4. They usually have high melting points and high boiling points as the strong ionic bonds require a lot of energy to break. 5. They usually dissolve in water but do not usually dissolve in organic solvents such as ethanol. ASSIGNMENT#1 In your notebook, draw diagrams (both methods) to show the bonding between: 1) Lithium and chlorine 2) Magnesium and oxygen 3) Aluminium and nitrogen 4) Calcium and chlorine 5) Sodium and sulphur 6) Aluminium and fluorine 7) Calcium and sulphur 8) Potassium and oxygen 9) Aluminium and chlorine 10) Magnesium and nitrogen N.B: Write the formula of the compound formed FIRST before drawing diagrams! COVALENT BONDING Covalent bonding is the sharing of electrons between non-metal atoms to form molecules. The shared pair of electrons is called a covalent bond. When electrons are shared, each atom ends up with the configuration of the noble gas nearest to it. Hydrogen atoms need a duplet of electrons, that is, 2 electrons to become stable. All other non-metal atoms need an octet of electrons (8 electrons) in their outermost shells to become stable. PROPERTIES OF COVALENT COMPOUNDS (e.g. carbon dioxide, CO 2) 1. They consist of molecules. 2. They are mostly liquids and gases. 3. They DO NOT conduct electricity in any state as they are no ions or free electrons present. 4. They have low melting points and boiling points. 5. They are usually soluble in organic solvents such as ethanol, but insoluble in water.

10 10 DRAWING BONDING DIAGRAMS IN COVALENT BONDING Example # 1 Draw diagrams (both methods) to show the bonding between 2 chlorine atoms. Two chlorine atoms hence the formula is Cl 2 (diatomic) Dot and Cross method Electronic shells A chlorine atom has an electronic configuration of since its atomic number is 17. It is a non-metal with seven valence electrons. It therefore needs one more electron to fill its outer shell and attain a noble gas configuration. During the formation of a chlorine molecule, two chlorine atoms come close

11 11 enough for their outer shell electrons to overlap. The two atoms share one pair of electrons between them, forming a single covalent bond. Please note the following: A double covalent bond is where two pairs of electrons are shared. A triple covalent bond is where three pairs of electrons are shared. MORE EXAMPLES... Now copy carefully into the spaces below more examples as your teacher goes through them on the board. Dot and Cross Bonding between two oxygen atoms Electronic shell Bonding between two nitrogen atoms Bonding between hydrogen and oxygen

12 12 Dot and Cross Bonding in CO 2 Electronic shell Bonding between carbon and fluorine ASSIGNMENT #2 In your notebook, draw diagrams (both methods) to show the bonding: 1. between 2 hydrogen atoms 2. between hydrogen and sulphur 3. between phosphorus and chlorine 4. in CS 2 5. between silicon and hydrogen 6. in SiO 2 7. between carbon and chlorine

13 13 So to sum up drawing diagrams in chemical bonding, here are some points to remember: Determine the number of atoms of each element involved by working out the formula of the compound using the valency approach. Is a metal atom involved? Metals have one, two or three valence electrons which will mean that the bonding will be electrovalent/ ionic (transfer of electrons). The diagram should consist of two parts. Make sure to show the charges of the ions formed and use square brackets for ions. Are all the elements involved non-metal atoms? Non-metal atoms have four, five, six or seven valence electrons hence the bonding will be covalent (sharing of electrons). Draw a diagram showing the overlap of the shared electrons. Note also that when drawing atoms, an atom having a configuration that involves three shells like sodium should be drawn larger than an atom that has a configuration that involves two shells like lithium 2.1. Always indicate the nucleus in the diagrams of the atoms. Assignment #3 Nitrogen, potassium, sulphur and fluorine are four elements with atomic numbers 7, 19, 16 and 9 respectively. 1. Define the terms (a) electrovalent bonding (b) covalent bonding 2. What is (a) an electrovalent bond (b) a covalent bond? 3. Deduce the formula of the compound formed between: (a) Nitrogen and fluorine (b) Potassium and sulphur 4. Draw diagrams (dot and cross or otherwise) to show the bonding between nitrogen and fluorine. 5. Draw diagrams (dot and cross or otherwise) to show the bonding between potassium and sulphur. 6. Give two properties of covalent compounds and two properties of electrovalent compounds. 7. Draw bonding diagrams for the following compounds:

14 14 CHEMICAL EQUATIONS Equations show substances involved in a chemical reaction. Balanced equations show how many of each kind of atom react together. Students must be able to find the number of each type of atom for example in each of the following compounds: 3NaOH contain 3 sodium atoms, 3 oxygen atoms and 3 hydrogen atoms. 5CO 2 contain 5 carbon atoms and 10 oxygen atoms. 2(NH 4) 2SO 4 contain 4 nitrogen atoms, 16 hydrogen atoms, 2 sulphur atoms and 8 oxygen atoms. Exercise In your notebook find the number of each type of atom in the following compounds: 1) 4Pb(NO 3) 2 2) 3CaCO 3 3) 5(NH 4) 3PO 4 4) 10C 6H 12O 6 5) 7C 2H 4 BALANCING EQUATIONS A chemical equation represents a chemical change (a chemical reaction) by means of symbols and formulae. Example: Fe + S FeS In words the equation above means that iron reacts with sulphur to produce iron (II) sulphide. The equation for the reaction between hydrogen and oxygen seems to be: H 2 + O 2 H 2O This is NOT a balanced equation because we have two oxygen atoms on the LHS and only ONE oxygen atom on the RHS. We can only balance an equation by putting a number IN FRONT of the ENTIRE FORMULA. We CANNOT change up the formulae just to make it balance: 2H 2 + O 2 2 H 2O RULES Write the formulae for the reactants on the left hand side and the formulae for the products on the right hand side of the side of the equation. e.g. Zn + HCl ZnCl 2 + H 2 Balance the equation (if necessary) by writing numbers IN FRONT of the formula e.g. e.g. Zn + HCl ZnCl 2 + H 2

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